NOx absorber catalysts (NACs) are known e.g. from U.S. Pat. No. 5,473,887 and are designed to adsorb nitrogen oxides (NOx) from lean exhaust gas (lambda>1) and to desorb the NOx when the oxygen concentration in the exhaust gas is actively—as opposed to passively—decreased. Such active decrease in oxygen concentration is known as “regeneration” of the NAC's NOx adsorption activity or a “purge” of NOx adsorbed on a NAC. Desorbed NOx may be reduced to N2 with a suitable reductant, e.g. gasoline fuel, promoted by a catalyst component, such as rhodium, of the NAC itself or located downstream of the NAC. In practice, oxygen concentration can be actively adjusted to a desired redox composition intermittently in response to a calculated remaining NOx adsorption capacity of the NAC, e.g. to richer than normal engine running operation (but still lean of stoichiometric, i.e. lambda=1 composition), to stoichiometric or to rich of stoichiometric (lambda<1). The oxygen concentration can be adjusted by a number of means, e.g. throttling, injection of additional hydrocarbon fuel into an engine cylinder such as during the exhaust stroke or injecting hydrocarbon fuel directly into exhaust gas downstream of an engine manifold.
A typical NAC formulation disclosed in the prior art includes a catalytic oxidation component, such as platinum, a significant quantity, i.e. substantially more than is required for a promoter, of a NOx-storage component, typically an alkali metal or alkaline earth metal such as barium, and a reduction catalyst, e.g. rhodium. One mechanism commonly given for NOx-storage from a lean exhaust gas for this formulation is:NO+½O2→NO2  (1); andBaO+NO2+½O2→Ba(NO3)2  (2),wherein in reaction (1), the nitric oxide reacts with oxygen on active oxidation sites on the platinum to form NO2. Reaction (2) involves adsorption of the NO2 by the storage material in the form of an inorganic nitrate.
At lower oxygen concentrations and/or at elevated temperatures, the nitrate species become thermodynamically unstable and decompose, producing NO and/or NO2 according to reaction (3) below. In the presence of a suitable reductant, these nitrogen oxides are subsequently reduced by carbon monoxide, hydrogen and hydrocarbons to N2, which can take place over the reduction catalyst (see reaction (4)).Ba(NO3)2→BaO+2NO+ 3/2O2 or Ba(NO3)2→BaO+2NO2+½O2  (3); andNO+CO→½N2+CO2  (4);(Other reactions include Ba(NO3)2+8H2→BaO+2NH3+5H2O followed by NH3+NOx→N2+yH2O or 2NH3+2O2+CO→N2+3H2O+CO2 etc.).
In the reactions of (1)-(4) above, the reactive barium species is given as the oxide. However, it is understood that in the presence of air most of the barium is in the form of the carbonate or possibly the hydroxide. The skilled person can adapt the above reaction schemes accordingly for species of barium other than the oxide and sequence of catalytic coatings in the exhaust stream, or for use with other alkaline earth metals such as Sr, Mg or Ca or alkali metals such as K or Cs.
Alkali metal-based NOx storage components have relatively high levels of NOx storage, so their use is desirable. However, there are a number of drawbacks in their use. These include migration of alkali metal from a catalyst into a ceramic monolith substrate on which the catalyst is coated, vaporisation of alkali metals during ageing of the catalyst in use, leaching of alkali metals by liquid water present in an exhaust system e.g. during engine cold-start, migration of alkali metals between layers in a catalytic washcoat and reduction of the hydrocarbon conversion efficiency by Pt (on this last drawback, see WO 02/22241).
The prior art discloses a number of NOx storage components. For example, U.S. Pat. No. 6,497,848 discloses a catalytic trap effective for conversion of NOx in an exhaust gas stream which is inert to high-temperature reaction with basic oxygenated compounds of lithium, sodium or potassium. The catalytic trap is substantially free of silica components and may include a catalytic trap material which contains a refractory metal oxide support, e.g., alumina, having dispersed thereon a catalytic component, such as a platinum group metal catalytic component, and a NOx sorbent comprised of one or more of the basic oxygenated compounds.
U.S. Pat. No. 6,727,202 discloses a catalytic trap comprising a catalytic trap material and a refractory carrier member on which the catalytic trap material is disposed. The catalytic trap material comprises: (i) a refractory metal oxide support; (ii) a catalytic component effective for promoting the reduction of NOx under stoichiometric or rich conditions; and (iii) a NOx sorbent effective for adsorbing the NOx under lean conditions and desorbing and reducing the NOx to nitrogen under stoichiometric or rich conditions. The NOx sorbent comprises a metal oxide selected from the group consisting of one or alkali metal oxides, alkaline earth metal oxides and mixtures of one or more alkali metal oxides and alkaline earth metal oxides. The manganese component is selected from the group consisting of: (1) a manganese oxide, (2) a mixed oxide of manganese and a transition metal and/or a rare earth metal, (3) a compound of an alkali metal and a manganese oxide, (4) a compound of an alkaline earth metal and a manganese oxide and (5) mixtures of the foregoing oxides and compounds. The combination in component (a) of an alkali metal oxide and silica or any siliceous compound is expressly excluded.
WO 97/02886 discloses a NOx abatement composition comprises a NOx abatement catalyst and a NOx sorbent material, which are dispersed in proximity to, but segregated from, each other on a common refractory carrier member. The NOx sorbent material comprises a basic oxygenated metal compound and optionally further comprises ceria. The NOx abatement catalyst contains a catalytic metal component including a platinum metal catalytic component. The catalytic metal component is segregated from the NOx sorbent material, which may be one or more of metal oxides, metal carbonates, metal hydroxides and mixed metal oxides. At least the catalytic metal component and the NOx sorbent material must be on, or comprise separate, particles; the particles may either be admixed or may be disposed in separate layers on the carrier member.
U.S. Pat. No. 5,451,558 discloses a method of decreasing the level of NOx, CO and SO2 emissions in a gas turbine. A catalyst absorber, preferably made of alumina/platinum/carbonate salt, is used to oxidize the pollutant oxides and absorb them.
US 2006/0035782 discloses a coarsening resistant automotive exhaust catalyst composition comprising a metal or metal-containing compound and a component having alkali metal or an alkaline-earth metal ions bonded to a conjugate base oxide of an inorganic acid. The inorganic acid for which the base oxide is conjugate has a Ka such that the automotive exhaust catalyst composition resists phase transitions that reduce surface area. The present invention also provides a NOx trap which incorporates the exhaust catalyst composition of the invention.
WO 02/22241 discloses a NOx-trap composition comprising: (a) at least one first NOx storage component comprising at least one alkali metal supported on at least one first support material; and (b) a platinum oxidation catalyst and at least one second NOx storage component not being an alkali metal supported on at least one second support material, whereby the platinum oxidation catalyst and the at least one alkali metal are physically segregated thereby susbtantially maintaining the hydrocarbon conversion activity of the platinum oxidation catalyst.
Historically, vehicular Diesel engines have been designed to meet some combination of four features: fuel efficiency; control of NOx emissions; power output; and particulate matter control. Early emission standards limited the quantity of carbon monoxide and hydrocarbon that it was permissible to emit, which forced fitment of Diesel oxidation catalysts to meet the standards. By the inception of Euro 5 emission standards were most easily met by tuning the engine to control NOx emissions and providing a filter in the exhaust system to trap particulate matter (the so-called NOx/particulate matter trade-off). For Euro 6 and proposals in the US, it appears to be difficult to meet particulate emission standards (which now include a requirement to reduce particulate number emissions) without a filter, whether additional catalyst-based NOx removal strategies are required in addition. Whilst it is possible, therefore, to conceive of an exhaust system comprising a NOx absorber catalyst on a flow-through substrate monolith in the absence of a filter for use in meeting e.g. European emission standards, generally we expect that a system comprising a NOx absorber catalyst on a flow-through substrate will be used in some combination with a filter, or that the NOx absorber catalyst will be coated on a filter substrate monolith, e.g. a wall-flow filter.
A typical exhaust system arrangement for a light-duty Diesel vehicle comprises a NOx absorber catalyst on a flow-through substrate monolith and a catalysed soot filter (CSF) disposed downstream (i.e. in the ordinary flow direction) thereof. Typical problems associated with NOx absorber catalyst development for use in such systems include NOx storage and NOx regeneration under low temperature, relatively high flow rate and relatively high hydrocarbon exhaust gas conditions. Modern Diesel vehicles generally use an engineering solution known as exhaust gas recirculation (EGR) in order better to control NOx emissions, wherein a portion of the exhaust gas is recirculated to the engine inlet during at least some of an internally programmed engine speed/load map. The point in the exhaust system from where the exhaust gas for EGR is taken contributes to the above problems. One typical arrangement is to take EGR exhaust gas from downstream of the CSF, so-called low pressure (or “long-loop”) EGR.